SOBOBA HOT SPRINGS, Calif. – The mother of one of the four firefighters who died battling a wildfire that authorities blamed on arsonists urged those who set it to turn themselves in Saturday.

"I firmly believe you didn't believe that things were going to turn out the way they did, but they did," said Bonnie McKay, whose son Jason, 27, died Thursday. "Don't let the remorse eat you alive. Come forward. ... I for one will try not to judge you. There is only one who can judge you."

Meanwhile, firefighters took advantage of calm weather and dissipating Santa Ana winds, making headway against the 63-square-mile conflagration by dumping water and retardant on flames using a fleet of helicopters and airplanes, including a DC-10 jumbo jet.

Still, forestry officials worried about the fire spreading in one area.

Scott McLean, spokesman for the California Department of Forestry, said the southeastern flank was threatening to spread to Black Mountain, a forested area even steeper than where it is burning now that is difficult to access.

"If it goes there, the fire is going to hell in a handbasket," said McLean from the command post in Beaumont, 90 miles east of Los Angeles.

The 40,450-acre blaze was 60 percent contained, two days after blowtorch gusts overran a U.S. Forest Service crew, killing four of its members and leaving a fifth clinging to life with burns over most of his body.

Investigators combed the area Saturday, looking for clues on how the fire engulfed the men so quickly. They interviewed a handful of firefighters who were nearby when it happened, Al Matecko, spokesman for a national investigating team.

A reward for information leading to the arsonist soared to $500,000 Friday, as $100,000 posted by Riverside County quickly multiplied with matching offers from the state, neighboring San Bernardino County, Rancho Mirage resident Tim Blixseth and the Morongo Band of Mission Indians.

Investigators were looking into whether the wildfire was related to other blazes in recent months, including a canyon fire last weekend, though a sheriff's spokesman said there was no immediate indication of a serial arsonist.

Residents said they saw two young men leaving the area where the fire broke out early Thursday west of the San Jacinto Mountains.

Fire officials said there had been six other minor injuries to firefighters and, after completion of damage assessments, raised the number of destroyed homes to 27, up from earlier estimates of 10.

Evacuation orders remained in effect for about 500 homes in Twin Pines and Poppet Flat, communities where homes burned. Residents were allowed back in for several hours to retrieve personal items and feed or remove animals.

Some found nothing to return to.

"There's nothing left, just a couple of walls and rubble," said Oscar Pineiro, 52, who had returned to his home in Twin Pines with his wife.

Forecasters predicted winds would ease and temperatures would drop slightly throughout the weekend, which could help as crews work to build firelines around the blaze.

The north side of the fire, paralleling Interstate 10, was considered well-contained. On the west flank, Highway 79 was reopened after firefighters stopped the fire's advance in that direction.

Major firefighting activity focused on the south side of the fire to prevent any southerly spread toward small communities including Soboba Hot Springs and San Jacinto.

Water-dropping helicopters operated from a landing zone in a grapefruit orchard on the Soboba Indian Reservation as retardant bombers flew overhead, and a half-dozen bulldozers lined up at the mouth of Castile Canyon, ready to move in to cut fire breaks.

Hotshot crews of reinforcements moved in as fire Battalion Chief Art Nevarez and his crew from neighboring Orange County came out of the canyon after a night of firefighting in what one called "goat country."

"We were real fortunate last night because the wind lay down for us," said Nevarez, who helped fight another major wildfire last month.

"When the Santa Anas come through, then it's really the big finale," he said of the fire season.

On Saturday, fire vehicles were gathered in the area and investigators walked with heads down as they looked for evidence around the ignition point near Esperanza Avenue.

One law enforcement official who spoke on condition of anonymity described the investigation as being in its "infancy."

Authorities declared the fire arson within hours of its start but have withheld details of any evidence they have. Fire officials have noted an unusual number of fires in the area in recent months, including one in a nearby canyon a week ago.